Certain clinical and morphologic observations are described in 10 patients who had mitral replacement (MVR) and lacerations of the left ventricular (LV) free wall midway between the anulus of the mitral valve and the stumps of the LV papillary muscles. In 5 patients, the lacerations led to LV free wall rupture, with immediate hemopericardium in 2 delayed (2-4 days) rupture in the other 3. Of the other 5 patients, 3 developed aneurysm of the LV free wall, the mouth of which was located midway between mitral anulus and papillary-muscle stumps, the sites of the lacerations observed in the other 7 patients. The remaining 2 patients had midway lacerations which produced neither rupture nor aneurysmal formation. The midway LV lacerations are considered the result of LV incisions made at the time of mitral valve excision.